1. Field of the Invention
This invention relates to a solid-state image pickup device comprising a charge coupled device (CCD) in the interline transfer system, and more particularly to a solid-state image pickup device in which the structure of a transfer electrode is improved, and a manufacturing method thereof.
2. Prior Art
FIG. 1 shows a top view of a conventional solid-state image pickup device comprising a charge coupled device in the interline transfer system. Particularly, FIG. 1 shows diagrammatically a part of a photosensing portion of the device, in which a p-type impurity layer (p-well) formed on the surface of a semiconductor substrate is provided with plural n-type impurity layers in a matrix formed to form plural photo-diodes Pd.sub.11 . . . , Pd.sub.1n, Pd.sub.21, . . . , Pd.sub.2n, Pd.sub.31, . . . , Pd.sub.3n, and so on in the p-type impurity layer. Plural charge channels L.sub.1, L.sub.2, L.sub.3 and so on (hereinafter referred to as "L.sub.j " (j=1, . . . , n)) for transferring signal charges in the vertical direction are formed between the adjacent columns of photo-diodes.
The hatched portion of FIG. 1 encircled with a dotted line, except for each photo-diode Pd.sub.ij and each charge transfer channel L.sub.j, illustrates a channel stopper (isolation).
Plural transfer electrodes G.sub.1, G.sub.2, G.sub.3 and so on (hereinafter referred to as "G.sub.i (i=1-m), each comprising a polysilicon layer extending horizontally, are placed in juxtaposition with each other on the charge transfer channel. The respective adjacent four electrodes of the transfer electrodes are provided with transfer clock signals .phi.1, .phi.2, .phi.3 and .phi.4 to be actuated by a four-phase driving system. That is, two adjacent electrodes (for example, G.sub.1 and G.sub.2), each partly overlapping the other, are assigned to the photo-diodes (for example, Pd.sub.11, Pd.sub.12, . . . , Pd.sub.1n) arranged in a row. Such partial overlap between the two adjacent electrodes prevent the occurrence of a potential barrier between the transfer electrodes and cause the charge transfer channels L.sub.3 to stably transfer signal charges, which are produced in photodiodes Pd.sub.ij and outputted through the transfer gates Tg.sub.11, Tg.sub.12, . . . , T.sub.gmn to the corresponding charge transfer channels L.sub.j.
The conventional solid-state image pickup device as described above has a disadvantage that a steep step portion formed at the overlapped portion of the respective adjacent transfer electrodes (e.g., a portion indicated by EJ shown in FIG. 2) is liable to be damaged, resulting in the degradation of the voltage-resistance of the transfer electrodes and decrease of yield of products, and further has another disadvantage that it is difficult to improve the degree of integration of the device in the direction of charge transfer because the overlapped portions as described above are required.